Purposes of Differential Coding
To demodulate BPSK one needs to make a local oscillator synchronous with the remote one. This is accomplished by a carrier recovery circuit. However, a carrier can be recovered in different ways, depending upon a valid phases count (2 for BPSK).
For this coding, if a carrier is recovered incorrectly, the received data are inverted.
Assuming that is a bit intended for transmission, and is a bit actually transmitted (differentially encoded), if
is transmitted, then on the decoding side
can be reconstructed, where indicates binary or modulo-2 addition.
Now depends only on a difference between and and not on their values. So, whether the data stream is inverted or not, the decoded data will always be correct.
When data is transmitted over twisted-pair wires, it is easy to accidentally insert an extra half-twist in the cable between the transmitter and the receiver. When this happens, the received data are inverted. There are several different line codes designed to be polarity insensitive -- whether the data stream is inverted or not, the decoded data will always be correct. The line codes with this property include differential Manchester encoding, bipolar encoding, NRZI, biphase mark code, coded mark inversion, and MLT-3 encoding.
Read more about this topic: Differential Coding
Famous quotes containing the words purposes of, purposes and/or differential:
“To be President of the United States, sir, is to act as advocate for a blind, venomous, and ungrateful client; still, one must make the best of the case, for the purposes of Providence.”
—John Updike (b. 1932)
“The new always happens against the overwhelming odds of statistical laws and their probability, which for all practical, everyday purposes amounts to certainty; the new therefore always appears in the guise of a miracle.”
—Hannah Arendt (1906–1975)
“But how is one to make a scientist understand that there is something unalterably deranged about differential calculus, quantum theory, or the obscene and so inanely liturgical ordeals of the precession of the equinoxes.”
—Antonin Artaud (1896–1948)